Fuel cells have been proposed as a power source for electric vehicles, stationary power supplies and other applications. One known fuel cell is the PEM (i.e., Proton Exchange Membrane) fuel cell, which is an acid-type fuel cell. Other types of fuel cells include alkaline-type, molten-carbonate-type and solid-oxide-type. A PEM fuel cell includes a so-called MEA (“membrane-electrode-assembly”) comprising a thin, solid polymer membrane-electrolyte (active element) having an anode on one face and a cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive contact elements which serve as current collectors for the anode and cathode, which may contain appropriate channels and openings therein for distributing the fuel cell's gaseous reactants (i.e., H2 and O2/air) over the surfaces of the respective anode and cathode. A diffusion media member is disposed between the MEA and each electrically conductive contact element. The diffusion media is operable to transport the fuel cell's gaseous reactants from the flow channels to the catalyst layers on the MEA. The diffusion media members require a compressive force to be exerted thereon to achieve a desired low electrical contact resistance to facilitate the collection of electrical current by the current collectors.
Fuel cell stacks typically comprise a plurality of fuel cells stacked one upon another. In PEM fuel cell stacks a plurality of the MEAs are stacked together in electrical series while being separated one from the next by an impermeable, electrically conductive contact element known as a bipolar plate or current collector. In some types of fuel cells each bipolar plate is comprised of two separate plates that are attached together with a fluid passageway (internal cavity) therebetween through which a coolant fluid flows to remove heat from both sides of the MEAs. In other types of fuel cells the bipolar plates include both single plates and attached together plates which are arranged in a repeating pattern with at least one surface of each MEA being cooled by a coolant fluid flowing through the two plate bipolar plates.
The plurality of stacked fuel cells form a fuel cell stack which is compressed to hold the plurality of fuel cells in a compressive relation. The fuel cell stack requires a significant amount of compressive force to squeeze the fuel cells of the stack together. The need for the compressive force comes about from the internal gas pressure of the reactants within the fuel cells plus the need to maintain good electrical contact between the internal components of the fuel cells.
During the manufacture of the fuel cell stack, the fuel cells are compressed to an initial compressive state. Subsequent operation of the fuel cell stack can cause the compression of the fuel cells to change. The change can occur in an overall compression of the entire fuel cell stack or in localized changes in the compressive force experienced by individual fuel cells or portions of a single fuel cell or adjacent fuel cells.
These changes in the compressive force imparted on the fuel cells and/or fuel cell stack can be detrimental. Excessive compressive forces can cause premature failure of individual fuel cells and/or the entire fuel cell stack. When the compressive forces are too low, the output of the individual fuel cells and/or the entire fuel cell stack can be degraded. Thus, it would be advantageous to compensate for the dimensional changes that occur within the fuel cell stack while maintaining the compressive force on the fuel cells and the fuel cell stack within a predetermined range.